1. Field of the Invention
The invention relates generally to cellular telephones and more particularly to a cellular telephone having a global positioning system (GPS) receiver using a blended combination of GPS signals from GPS satellites and GPS pseudolites for determining a location.
2. Description of the Prior Art
Recently, the Federal Communications Commission (FCC) of the United States has issued an E911 requirement for cellular phones mandating that it be possible to determine the location of the phone to within one-hundred twenty-five meters within about five seconds some high percentage of the time. This requirement might conceivably be met by a global positioning system (GPS) receiver in a cellular phone. GPS receivers have been used for several years for determining geographical location and/or time in commercial applications including navigation, timing, mapping, surveying, machine and agricultural control, vehicle tracking, and marking locations and time of events. Given such wide commercial application, it is clear that GPS receivers provide a good value for many users. However, there are several issues that must be resolved for the use a GPS receiver in a cellular telephone.
A GPS receiver receives signals from GPS satellites operated by the United States government. The GPS satellites orbit the Earth in known orbits at an altitude of about ten thousand miles. Because of the great distance they must travel, the GPS signals have relatively low signal levels by the time they reach the GPS receiver on or near the surface of the Earth. Obviously, existing GPS receivers are capable of operation with these low signal levels. However, in order to receive these low signal levels, GPS receivers typically use specially designed GPS antennas that are positioned to point upward with a clear line-of-sight to the GPS satellites. For use with a handheld cellular telephone, the GPS antenna might be permanently mounted on the outside of an automobile or worn by a user on a hat or shoulder pad and then somehow connected to the cellular telephone. However, it is not expected that such an approach would be acceptable by users except in special circumstances.
An even more significant issue for a GPS receiver in a cellular telephone is that signals in the microwave range of the GPS signal are severely attenuated by passing through or around obstructions such as the roofs or walls of buildings or automobiles. Typically, this is not a problem where the GPS receiver is mounted on a platform such as a ship, airplane, farm tractor, or a vehicle traveling on an open highway. However, the additional attenuation has to date prevented reliable operation for GPS receivers located within buildings and significantly limited operation in canyons, urban city blocks, and behind heavy foliage. It is not expected that a cellular telephone location system that is limited to outdoor use would be generally acceptable by users or would meet FCC requirements.
Experiments have been made using pseudolites for augmenting the satellite constellation and thus improving availability of the GPS signal. Typically, the pseudolites are fixed on the ground. The pseudolites broadcast pseudolite GPS signals that mimic the satellite GPS signals by including location-determination information that is appropriate to the geographical location of the pseudolite. When the GPS pseudolite is nearby, a relatively high signal level is obtained even within buildings, thereby nearly guaranteed signal reception of a GPS receiver. However, existing GPS receivers are not expected to be capable of operation with the pseudolite GPS signals due to differences between the satellite and pseudolite GPS signals.
There is a need for a cellular telephone having a GPS receiver capable of receiving pseudolite GPS signals in order to augment the satellite GPS signals and enable the cellular telephone to determine its location when satellite GPS signals are blocked by local obstructions.
It is therefore an object of the present invention to provide a cellular telephone having an internal GPS receiver for determining and reporting location using satellite GPS signals transmitted by GPS satellites and pseudolite GPS signals transmitted by GPS pseudolites.
Another object of the present invention is to provide ancillary equipment for the cellular telephone system for transmitting acquisition aiding information to a cellular telephone having an internal GPS receiver for achieving a fast time to first fix.
Briefly, in a preferred embodiment, a cellular telephone system of the present invention includes at least one remote cellular telephone, a cell site transceiver, global positioning system (GPS) satellites, and GPS pseudolites. The cellular telephone includes a cellphone transceiver and a GPS receiver. The cellular transceiver receives wireless downlink cellphone signals from the cell site transceiver and transmits wireless uplink cellphone signals to the cell site transceiver. The cell site transceiver passes voice and data from the downlink and uplink cellphone signals within a local area into the worldwide network of telephone services. The GPS receiver receives satellite GPS signals from GPS satellites and pseudolite GPS signals from GPS pseudolites. The GPS satellites broadcast satellite GPS signals having location-determination information that can be used by a GPS receiver anywhere in the world for determining a location of the GPS receiver. The GPS pseudolites augment the GPS satellites for the local area by broadcasting pseudolite GPS signals having location-determination information in a format that is similar to the format for the satellite GPS signals. The GPS pseudolites can be located anywhere nearby, including on a cell site tower with the cell site transceiver. When the GPS pseudolite is in the same local area as the GPS receiver, the relatively high power level of pseudolite GPS signals increases the probability of reception in spite of local obstructions. The GPS receiver uses either the satellite GPS signals, the pseudolite GPS signals, or a blended combination of the satellite and pseudolite GPS signals for determining its location. Then, the cellphone transceiver reports the location to the cell site transceiver in the uplink cellphone signal. Optionally, the cellular telephone stores its location for other uses such as navigation and mapping.
The cell site transceiver transmits a downlink cellphone signal having acquisition aiding information optionally including GPS-based time; ephemeris orbital parameters for GPS satellites; and pseudorandom (PRN) codes, locations, and signal characteristics used by GPS pseudolites in the local vicinity. The GPS receiver in the cellular telephone uses the acquisition aiding information for speeding the time to the first location fix.
An advantage of a cellular telephone of the present invention is that pseudolite GPS signals are used for a high probability of determining a GPS-based location when satellite GPS signals are blocked by local obstructions such as buildings, automobiles, canyons, or heavy foliage.
Another advantage of a cellular telephone of the present invention is that a first GPS-based location fix is obtained quickly.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.